1. Field of the Invention The invention relates to a GPS receiving antenna that receives GPS signals transmitted from a GPS satellite.
2. Description of the Related Art
In recent years, a system to receive signal waves transmitted from multiple artificial satellites that orbit around the earth by a receiver and detect the present position of the receiver based on information included in the received signal waves has come into widespread use. The system is generally called GPS (Global Positioning System) in countries including Japan and the United States of America and typically uses the GPS satellites controlled by the U. S. Department of Defense, while there are similar systems such as Galileo in Europe and Glonass in the Russian Federation. Herein, the positioning system using artificial satellites, the artificial satellites for the positioning system, signal waves transmitted from the artificial satellites, and receivers receiving the signal waves will be referred to as GPS, GPS satellites, GPS signals, and GPS receivers, respectively for ease of representation.
The GPS allows the present position of a moving body to be detected highly accurately and almost in real time, and therefore the system is primarily used for measuring the present position of a moving body such as an automobile, an airplane and a mobile telephone using a receiver provided in the moving body.
Today, GPS receivers suitable for automobiles, in other words, vehicle GPS receivers have rapidly come into widespread use. When such a GPS receiver is provided in an automobile, an antenna device for receiving a GPS signal is provided on the exterior of the automobile such as on the roof.
As shown in FIG. 7, a conventional antenna device 100 includes an antenna case 103 having a top cover 101 and a bottom plate 102 joined with each other, an antenna module 104 stored in the top cover 101, and a packing member 105 provided at the joining part between the top cover 101 and the bottom plate 102 for keeping the antenna case 103 tightly sealed, and a signal line 106 connected to the antenna module 104.
The antenna module 104 includes an antenna element 110 having an antenna for receiving a GPS signal transmitted from a GPS satellite, a circuit board 111 having a circuit for carrying out various kinds of signal processing such as signal amplification to the GPS signal received at the antenna element 110. The antenna element 110 and the circuit board 111 are joined with each other using for example a length of double-faced adhesive tape 112.
The circuit board 111 is connected with a signal line 113 for extracting the GPS signal to the outside of the antenna case 103. The circuit board 111 is provided with a shield case 114 for shielding the circuit on its surface opposite to the side of the antenna element 110. The signal line 113 is externally extended through a notch 101a formed in the top cover 101, and a gasket 115 is attached in a position corresponding to the notch 101a. 
In the antenna device 100, the antenna module 104 and the packing member 105 are stored in the internal space of the top cover 101, while the top cover 101 and the bottom plate 102 are integrally joined by securing four screws 120.
The bottom plate 102 has two recesses 120a (only one of which is shown in FIG. 7) and a permanent magnet (not shown) is provided in each of the recesses 120a. These permanent magnets are provided to secure the antenna 100 to the roof of the automobile by their magnetic attraction. At the main surface of the bottom plate 102 facing the outside, a thin aluminum name plate 121 having the model number and name of the antenna device 100 and the like thereon is provided. A transparent resin sheet 122 to protect the automobile roof against damage is attached over the nameplate 121 and on about the entire surface of the main surface (see Japanese Patent Laid-Open No. 2001-68912).
Now, the conventional antenna 100 having the structure described above is produced by assembling 17 parts altogether. In recent years, there has been a demand for reduced cost in the field of such receiving antennas, and therefore the number of parts has been reduced or the assembly process has been simplified for the purpose of reducing the product cost.
In the conventional antenna device 100, the top cover 101 and the bottom plate 102 are secured by the screws 120, and therefore the boss part or rib part to secure the screws 120 to the top cover 101 and the bottom plate 102 are necessary. In the conventional antenna device 100, the space for forming these boss part and rib part should be secured, and therefore there is a limit to the size reduction of the device as a whole.